Earth Facts: Interesting Facts about Planet Earth • The Planets Interactive Solar System Model Earth Facts Earth is the third planet from the Sun and largest of the terrestrial planets . Surprisingly, while it is only the fifth largest planet in terms of size and mass, it is the densest (5,513 kg/m3) of all the planets. Earth is the only planet in the solar system not named after a mythological being. Instead, its name is derived from the Old English word "ertha" and the Anglo-Saxon word "erda" which means ground or soil. Earth was formed somewhere around 4.54 billion years ago and is currently the only known planet to support life - and lots of it. Size of the Earth compared to the Moon Side by side comparison of the size of Earth vs the Moon Facts about Earth The Earth was once believed to be the centre of the universe. For 2000 years ancient astronomers believed that the Earth was static and had other celestial bodies travelling in circular orbits around it. They believed this because of the apparent movement on the Sun and planets in relation to their viewpoint.

In 1543, Copernicus published his Sun-centered model of the Solar System which put the Sun at the centre of our solar system. Earth is the only planet not named for a mythological god or goddess. The other seven planets in the solar system were named after Roman gods or goddesses. For the five visible to the naked eye, Mercury , Venus , Mars , Jupiter and Saturn they we named during ancient times. This Roman method was also used after the discovery of Uranus and Neptune . The word “Earth” comes from the Old English word “ertha” meaning ground or land. Earth is the most dense planet in the solar system. The density of Earth differs in each part of the planet – the core, for example, is denser than the Earth’s crust – but the average density of the planet is around 5.52 grams per cubic centimetre. The gravity between the Earth and the Moon causes the tides on Earth. This effect on the Moon means it is tidally locked to Earth – its rotation period is the same as its orbit time so it always presents the same face to Earth. The rotation of the Earth is gradually slowing down.

The deceleration of the Earth’s rotation is very slow, approximately 17 milliseconds per hundred years. Eventually this will lengthen our days but it will take around 140 million years before our day will have increased from 24 to 25 hours. Earth’s atmosphere is composed of 78% nitrogen, 21% oxygen, and trace amounts of other gases including argon and carbon dixoide. The large amount of oxygen on Earth comes from our plant life’s consumption of carbon dioxide during photosynthesis. Earth has a very powerful magnetic field. This field protects the planet from the effects of solar winds and is believed to be a result of the nickel-iron core of the planet combined with its rapid rotation. The Earth has an Ozone Layer which protects it from harmful solar radiation. This shell is a special type of oxygen that absorbs most of the Sun’s powerful UV rays. 70% of the Earth’s surface is covered by water – the remainder consists of continents and islands which together have many lakes and other sources of water. The first life on Earth developed in the oceans through a process called abiogenesis or biopoiesis. This is a natural process in which life grows from non-living matter like simple organic compounds.

Earth’s water was initially trapped within the planet. Over time the Earth’s water was brought to the surface by the planet’s volcanic activity. Earth has relatively few visible impact craters compared with other solid bodies in our solar system. This is because Earth is geologically active and has processes like tectonics and erosion that reshape its surface. The highest point found on Earth is Mount Everest which reaches a height of 8.8 km. The lowest point on Earth is called Challenger Deep and at 10.9 km below sea level, it is further than the peak of Mount Everest. Earth has one of the most circular orbits of all the eight planets. Its axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, which produces the seasons we experience. A year on Earth lasts just over 365 days. It is actually 1/4 of a day over 365 days which is why we have a leap year every four years. More information and facts about Earth Throughout human history we have sought to understand our home planet. However, the learning curve has been steep, with many errors having been made along the way.

For example, it was not until the time of the ancient Romans that the world was understood to be spherical rather than flat. A second example is the belief that the Sun revolved around the Earth. Only in the sixteenth-century, through the work of Copernicus, did we accept that, in fact, the Earth was merely a planet orbiting the Sun. Perhaps most importantly, it is during the last two centuries that science has allowed us to see that the Earth is both an ordinary and unique place in the Solar System . On one hand, many of its characterisitcs are rather unexceptional. Take, for example, its size, interior, and geological processes&#8212being the fifth largest out of the eight planets, it is close to the median in terms of size; its interior structure is almost identical to the three other terrestrial planets; and the same geological processes that shape its surface can be found not only on other planets, but also on planetary moons. However, the Earth is special in one very important regard&#8212in all of the solar system, the Earth is the only world known to foster life. Atmosphere The ability for Earth to possess life is dependent in many ways on its atmosphere.

The composition of the atmosphere is roughly 78% nitrogen (N2), 21% oxygen (O2), 1% argon, with trace amounts of carbon dioxide (CO2) and other gases. Nitrogen and oxygen are essential to DNA and biological energy production, respectively, without which life could not be sustained. Additionally, the oxygen found in what is known as the ozone layer of the atmosphere protects the surface of the planet by absorbing harmful solar radiation. Remarkably, the significant amount of oxygen present in the atmosphere is due to the life found on Earth. As a byproduct of making sugars, plants convert the carbon dioxide in the atmosphere into oxygen. Essentially, this means that without plants the amount of carbon dioxide in the atmosphere would be much greater and the oxygen levels much lower. On one hand, if carbon dioxide levels were much higher, it is likely the Earth would suffer from a runaway greenhouse effect like that on Venus . On the other hand, if the percentage of carbon dioxide were any lower there would not be a greenhouse effect at all, thus making temperatures far colder. Therefore, the carbon dioxide levels are just right to maintain hospitable temeperatures ranging from -88° C to 58° C.

Oceans When viewing Earth from space, there is one overwhelming feature&#8212the oceans of liquid water. In terms of surface area, these oceans cover approximately 70% of the Earth. What is even more amazing than this percentage is that a single drop of liquid water is yet to be found on any other planet in the Solar System. In this regard, the Earth is truly unique. Like the Earth’s atmosphere, the presence of liquid water is vital for life. In fact, life is believed to have first developed 3.8 billion years ago in the oceans, only later evolving the ability to survive on land. The existence of the oceans is attributed to two sources. The first of these is the Earth itself. It is conjectured that large amounts of water vapor were trapped wintin the Earth during its formation. Over time, the planet’s geological mechanisms, primarily its volcanic activity, released this water vapor into the atmosphere. Once in the atmosphere, this vapor condensed and fell to the planet’s surface as liquid water. The second source is theorized to have originated from the ancient comets that struck the Earth. Upon impact, they deposited substantial amounts of water ice on the planet.

Surface Although most of the Earth’s surface lies beneath its oceans, the remaining “dry” surface is quite remarkable. When comparing the Earth to other solid bodies in the Solar System, its surface stands out due to its lacking impact craters. It is not that the Earth has been spared the numerous impacts by small bodies; rather, it is because the evidence of these impacts has been erased. Although there are many geological processes responsible for this, the two most important are weathering and erosion. In many ways these two mechanisms can be thought of as working in tandem. Weathering is the breaking down of surface structures into smaller pieces by the atmosphere. Moreover, there are two types of weathering: chemical and physical. An example of chemical weathering is acid rain. An example of physical weathering is abrasion of river beds caused by rocks suspended in flowing water. The second mechanism, erosion, is simply the movement of weathered particles by water, ice, wind or gravity. Thus, impact craters have been “smoothed out” through weathering and erosion by being broken apart and redistributed to other areas on the Earth’s surface. Two other geological mechanisms have helped to shape the Earth’s surface. The first is volcanic activity.

This process consists of the releasing of magma (molten rock) from the Earth’s interior through a rupture in the its crust. Some effects of volcanic activity can be the resurfacing of Earth’s crust or formation of islands (think of the Hawaiian Islands). The second mechanism is orogeny, or the formation of mountains through the compression of tectonic plates. An example of mountains created through this process is the Rocky Mountains. Interior Similar to the other terrestrial planets, Earth’s interior is believed to consist of three components: a core, a mantle, and a crust. At present, the core is thought to be comprised of two separate layers&#8212an inner core composed of solid nickel and iron, and an outer core composed of molten nickel and iron. The mantle is very dense and almost entirely solid silicate rock; its thickness is roughly 2,850 km. Finally, the crust is also composed of silicate rock and varies in thickness. While the continental crust ranges from 30 to 40 km in thickness, the oceanic crust is much thinner at only 6 to 11 km.

Yet another distinguishing feature of the Earth when compared to the other terrestrial planets is that its crust is divided into cool, rigid plates that rest upon the hotter mantle below. Furthermore, these plates are in constant motion. Along the boundaries of these plates two processes, known as subduction and spreading, can occur. During subduction two plates come into contact (sometimes violently, producing earthquakes) and one plate is forced under the other. Separation, on the other hand, is when two plates are moving away from each other. Orbit & Rotation At roughly 365 days, the Earth’s orbit around the Sun is familiar to us. The length of our year is due in large part to the Earth’s average orbital distance of 1.50 x 108 km. What many people are not familiar with is that at this orbital distance it takes sunlight, on average, about eight minutes and twenty seconds to reach the Earth. With an orbital eccentricity of .0167, the Earth’s orbit is one of the most circular in all the Solar System. This means that the difference between Earth’s perihelion and aphelion is relatively small.

As a result of this small difference, the intensity of the sunlight Earth receives remains almost constant year-round. However, the Earth’s position in its orbit is responsible, in part, for the varying seasons it experiences. The Earth’s axial tilt is approximately 23.45°. That is, the axis the Earth rotates about is tilted slightly with respect to the plane in which the Earth orbits the Sun. The effect of this tilt, along with position of the Earth in its orbit, means that at certain times the amount of sunlight the northern hemisphere receives is greater than that of the southern hemisphere, and vice versa. This variation in intensity is what produces the warmer temperatures during the summer and colder temperatures during the winter. A second commonly know charecteristic is that the Earth takes approximately twenty-four hours to complete one rotation. This is fastest among the terrestrial planets, but easily slower than that of all the gas planets . Earth Profile Fun Jupiter Facts for Kids - Amazing Information About This Big Planet Check out our cool range of planet facts for kids. Enjoy amazing trivia and have fun learning about the planets found in our Solar System. Jupiter Facts for Kids Jupiter is the largest planet in our solar system.

Its famous 'Red Spot' and raging gas storms give it an impressive if not intimidating appearance. As well as having many moons, Jupiter also has a number of rings similar to that of Saturn but much less noticeable. Jupiter is the largest planet in our Solar system. It is so big that more than 1300 Earths could fit inside it. Thick, colorful clouds of deadly poisonous gases surround Jupiter. The quick spinning of the planet whips up the atmosphere, creating the bands around the planet. If you were to descend into Jupiter, the thin, cold atmosphere becomes thicker and hotter, gradually turning into a thick, dark fog. In the blackness about 1000km down the pressure squeezes the atmosphere so hard that it becomes like liquid. At the centre of Jupiter is a rocky core, slightly bigger than Earth but weighing about 20 times more. Surrounding the core is an ocean of liquid hydrogen, about 1,000 kilometres deep. Jupiter has many storms raging on the surface, most notably the big red spot which is the largest hurricane in our Solar System. It's been raging for over three hundred years.

Jupiter has a very strong magnetic field, you would weigh two and a half times as much as you would on Earth. Jupiter has many moons circling around it. Four of these moons are bigger than Pluto. Mass of Jupiter - Universe Today Universe Today by Jerry Coffey The mass of Jupiter is 1.9 x 1027 kg. It is hard to fully understand a number that large, so here are a few comparisons to help. It would take 318 times Earth’s mass to equal Jupiter’s. Jupiter is 2.5 times more massive than all of the other planets in our Solar System combined. Jupiter is actually so massive that if it gained much more mass it would shrink. How can additional mass cause a planet to shrink? Gravitational compression. Given that there is no more hydrogen or helium gas floating around for Jupiter to collect, it would gain mass through the accretion of rocky bodies like asteroids. Jupiter’s intense gravity would pull additional rock tightly together shrinking the diameter of the planet and increasing its density. As the density increased so would the gravity, further compressing the planet. Scientists estimate that Jupiter would have to accumulate 3-4 times its current mass in order to begin compressing.

Since there isn’t that much material in our Solar System, it is a pretty good bet that Jupiter will never shrink. But what if it did? There have been rumors floating around for decades that Jupiter could ignite fusion and become a star at any time. They are all false science and garbage at best. The mass of Jupiter is no where near enough for sustained nuclear fusion. Fusion requires high temperatures, intense gravitational compression, and fuel. Jupiter has the right fuels in abundance. There is plenty of hydrogen to be had, but the planet is too cold and lacks the density for a sustained reaction process. Scientists estimate that Jupiter needs 50-80 times its current mass to ignite fusion. As stated in the paragraph above, there isn’t enough material to be had, so Jupiter can not become a star. At one time, scientists thought that Jupiter was the largest that a planet could become without igniting fusion and becoming a star. They discovered the fallacy of that belief once technology expanded their view of the universe. According to Dr.

Sean Raymond, a post doctoral researcher at the Center for Astrophysics and Space Astronomy (CASA) at the University of Colorado, ”In terms of gaseous planets, once they reach 15 Jupiter masses or so there is enough pressure in the core to ignite deuterium fusion, so those are considered ‘brown dwarfs’ rather than planets.” As you can see, the mass of Jupiter may seem awesome in comparison to Earth, it is a tiny fish in a world of sharks. Scientists have found a few hundred gas giants larger than Jupiter as they have surveyed the night sky. Who knows what is in store as telescope technology improves in the coming years. Here’s an article from Universe Today explaining just how big planets can get , and an article about how Jupiter and the other gas giants might have gobbled up their moons while they were forming. This site has more detailed information about the mass of Jupiter, and a page from NASA that helps you calculate the density of the planets. We’ve also recorded an entire show just on Jupiter for Astronomy Cast. Listen to it here, Episode 56: Jupiter , and Episode 57: Jupiter’s Moons .

Sources: Neptune's Atmosphere: Composition, Climate & Weather Neptune's Atmosphere: Composition, Climate & Weather By Nola Taylor Redd, Space.com Contributor | December 14, 2012 05:46pm ET MORE The eighth and last planet in the solar system, Neptune has an atmosphere more comparable with Uranus than with Saturn and Jupiter. The two most distant planets boast atmospheres dominated by ices. But even with a chill in the air, Neptune still manages to host some of the most extreme and violent weather in the solar system. Neptune's Great Dark Spot, accompanied by white high-altitude clouds, as seen by a Voyager spacecraft. Credit: NASA/JPL-Caltech Atmospheric composition Neptune's atmosphere is made up predominately of hydrogen and helium, with some methane. The methane is part of what gives Neptune its brilliant blue tint, as it absorbs red light and reflects bluer colors. Uranus also has methane in its atmosphere , but has a duller shading. Something else must be contributing to Neptune's hue, but scientists aren't certain what. The planet has ten to a hundred times more methane, ethane, and ethyne at its equator than it does at its poles.

Like other gas giants, Neptune still has much of the same atmosphere it captured at its formation . Atmospheric composition by volume: Hydrogen Deuteride: 192 parts per million Ethane: 1.5 parts per million Ammonia ice, water ice, ammonia hydrosulfide, and methane ice also compose Neptune 's atmosphere. Layering the atmosphere The atmosphere of Neptune is made up of two main regions. Like the other three gas giants , the planet has no firm surface, so scientists have established that the "surface" is where the pressure is equal to the pressure found at sea level on Earth. Just above the surface lies the troposphere. As altitude increases, temperature in the troposphere decreases. But in the next layer, the stratosphere, temperatures increase with altitude. This is related to the motion inside of the planet's core, which heats Neptune more than the rays from the distant sun. The next layer is the thermosphere, where pressures are lower. The very outer edge of the atmosphere is known as the exosphere. Cloud patterns on Neptune The clouds of Neptune vary with the altitude. Cold temperatures allow methane clouds to condense in the highest layers of the atmosphere.

Scientists think that clouds made up of ammonia and hydrogen sulfide exist at higher pressures. Farther down, clouds of hydrogen sulfide, ammonium sulfide, ammonia, and water could exist. Clouds of water-ice may be found at pressures of 50 bars, with clouds of hydrogen sulfide and ammonia beneath them. The highest layers contain cirrus clouds made up of frozen methane, which have been observed casting shadows on other clouds 35 miles (56 kilometers) beneath them. Neptune also contains a haze at very high altitudes. These smog-like clouds are made up of hydrocarbons, much like smog over major cities on Earth. Powerful storms With winds of 1,305 miles per hour (2,100 km per hour), Neptune has some of the most extreme weather in the solar system. These winds travel three times as faster as those on Jupiter, and nine times faster as winds clocked on Earth. Such winds drive powerful storms. When Voyager 2 observed the planet in 1989, it tracked a dark, oval storm that resembled Jupiter's Great Red Spot. The enormous weather pattern was about the size of Earth.

However, while Jupiter's famous feature has lasted for hundreds of years, Neptune's first observed spot had disappeared when imaged again a few years later. Instead, the planet seems to boast a similar anticyclonic storm, with winds traveling backwards in high-pressure systems, every few years. Clouds are generated at or just below the tropopause region, which lies between the troposphere and the stratosphere. Neptune's weather is driven in part by heating from within its core. The planet has a temperature similar to Uranus', despite being half again as far from the sun, because Uranus has some peculiarities with its internal heating that may be a result of a collision early in its life. — Nola Taylor Redd, SPACE.com Contributor Related: What are the Different Masses of the Planets? - Universe Today Universe Today What are the Different Masses of the Planets? Article Updated: 22 Jul , 2016 by Matt Williams It is a well known fact that the planets of the Solar System vary considerably in terms of size. For instance, the planets of the inner Solar System are smaller and denser than the gas/ice giants of the outer Solar System .

And in some cases, planets can actually be smaller than the largest moons. But a planet’s size is not necessarily proportional to its mass. In the end, how massive a planet is has more to do with its composition and density. So while a planet like Mercury may be smaller in size than Jupiter’s moon Ganymede or Saturn’s moon Titan , it is more than twice as massive than they are. And while Jupiter is 318 times as massive as Earth, its composition and density mean that it is only 11.21 times Earth’s size. Let’s go over the planet’s one by one and see just how massive they are, shall we? Mercury: Mercury is the Solar System’s smallest planet, with an average diameter of 4879 km (3031.67 mi). It is also one of its densest at 5.427 g/cm3, which is second only to Earth. As a terrestrial planet , it is composed of silicate rock and minerals and is differentiated between an iron core and a silicate mantle and crust. But unlike its peers (Venus, Earth and Mars), it has an abnormally large metallic core relative to its crust and mantle.

All told, Mercury’s mass is approximately 0.330 x 1024 kg, which works out to 330,000,000 trillion metric tons (or the equivalent of 0.055 Earths). Combined with its density and size, Mercury has a surface gravity of 3.7 m/s² (or 0.38 g). Internal structure of Mercury: 1. Crust: 100–300 km thick 2. Mantle: 600 km thick 3. Core: 1,800 km radius. Credit: MASA/JPL Venus: Venus , otherwise known as “Earth’s Sister Planet”, is so-named because of its similarities in composition, size, and mass to our own. Like Earth, Mercury and Mars, it is a terrestrial planet, and hence quite dense. In fact, with a density of 5.243 g/cm³, it is the third densest planet in the Solar System (behind Earth and Mercury).

Its average radius is roughly 6,050 km (3759.3 mi), which is the equivalent of 0.95 Earths. And when it comes to mass, the planet weighs in at a hefty 4.87 x 1024 kg, or 4,870,000,000 trillion metric tons. Not surprisingly, this is the equivalent of 0.815 Earths, making it the second most massive terrestrial planet in the Solar System. Combined with its density and size, this means that Venus also has comparable gravity to Earth – roughly 8.87 m/s², or 0.9 g. Earth: Like the other planets of the inner Solar System, Earth is also a terrestrial planet, composed of metals and silicate rocks differentiated between an iron core and a silicate mantle and crust. Of the terrestrial planets, it is the largest and densest, with an average radius of 6,371.0 km (3,958.8 mi) and a mean of density of 5.514 g/cm3.

The Earth’s layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com And at 5.97 x 1024 kg (which works out to 5,970,000,000,000 trillion metric tons) Earth is the most massive of all the terrestrial planets. Combined with its size and density, Earth experiences the surface gravity that we are all familiar with – 9.8 m/s², or 1 g. Mars: Mars is the third largest terrestrial planet, and the second smallest planet in our Solar System. Like the others, it is composed of metals and silicate rocks that are differentiated between a iron core and a silicate mantle and crust. But while it is roughly half the size of Earth (with a mean diameter of 6792 km, or 4220.35 mi), it is only one-tenth as massive. In short, Mars has a mass of 0.642 x1024 kg, which works out to 642,000,000 trillion metric tons, or roughly 0.11 the mass of Earth.

Combined with its size and density – 3.9335 g/cm³ (which is roughly 0.71 times that of Earth’s) – Mars has a surface gravity of 3.711 m/s² (or 0.376 g). Jupiter: Jupiter is the largest planet in the Solar System. With a mean diameter of 142,984 km, it is big enough to fit all the other planets (except Saturn) inside itself, and big enough to fit Earth 11.8 times over. But with a mass of 1898 x 1024 kg (or 1,898,000,000,000 trillion metric tons), Jupiter is more massive than all the other planets in the Solar System combined – 2.5 times more massive, to be exact. Jupiter’s structure and composition. (Image Credit: Kelvinsong CC by S.A. 3.0) However, as a gas giant, it has a lower overall density than the terrestrial planets. It’s mean density is 1.326 g/cm, but this increases considerably the further one ventures towards the core.

And though Jupiter does not have a true surface, if one were to position themselves within its atmosphere where the pressure is the same as Earth’s at sea level (1 bar), they would experience a gravitational pull of 24.79 m/s2 (2.528 g). Saturn: Saturn is the second largest of the gas giants; with a mean diameter of 120,536 km, it is just slightly smaller than Jupiter. However, it is significantly less massive than its Jovian cousin, with a mass of 569 x 1024 kg (or 569,000,000,000 trillion metric tons). Still, this makes Saturn the second most-massive planet in the Solar System, with 95 times the mass of Earth. Much like Jupiter, Saturn has a low mean density due to its composition. In fact, with an average density of 0.687 g/cm³, Saturn is the only planet in the Solar System that is less dense than water (1 g/cm³). But of course, like all gas giants, its density increases considerably the further one ventures towards the core.

Combined with its size and mass, Saturn has a “surface” gravity that is just slightly higher than Earth’s – 10.44 m/s², or 1.065 g. Diagram of Saturn’s interior. Credit: Kelvinsong/Wikipedia Commons Uranus: With a mean diameter of 51,118 km, Uranus is the third largest planet in the Solar System. But with a mass of 86.8 x 1024 kg (86,800,000,000 trillion metric tons) it is the fourth most massive – which is 14.5 times the mass of Earth. This is due to its mean density of 1.271 g/cm3, which is about three quarters of what Neptune’s is. Between its size, mass, and density, Uranus’ gravity works out to 8.69 m/s2, which is 0.886 g. Neptune: Neptune is significantly larger than Earth; at 49,528 km, it is about four times Earth’s size.

And with a mass of 102 x 1024 kg (or 102,000,000,000 trillion metric tons) it is also more massive – about 17 times more to be exact. This makes Neptune the third most massive planet in the Solar System; while its density is the greatest of any gas giant (1.638 g/cm3). Combined, this works out to a “surface” gravity of 11.15 m/s2 (1.14 g). As you can see, the planets of the Solar System range considerably in terms of mass. But when you factor in their variations in density, you can see how a planets mass is not always proportionate to its size. In short, while some planets may be a few times larger than others, they are can have many, many times more mass. For more information, check out Nine Planets overview of the Solar System, NASA’s Solar System Exploration , and use this site to find out what you would weigh on other planets . Astronomy Cast has episodes on all of the planets. Here’s Episode 49: Mercury to start!

Share this: Planet Earth: Facts About Its Orbit, Atmosphere & Size Planet Earth: Facts About Its Orbit, Atmosphere & Size By Charles Q. Choi, Space.com Contributor | December 11, 2014 10:49pm ET MORE Credit: NASA Earth, our home, is the third planet from the sun. It is the only planet known to have an atmosphere containing free oxygen, oceans of liquid water on its surface, and, of course, life. Earth is the fifth largest of the planets in the solar system — smaller than the four gas giants, Jupiter , Saturn , Uranus and Neptune , but larger than the three other rocky planets, Mercury , Mars and Venus . Earth has a diameter of roughly 8,000 miles (13,000 kilometers), and is round because gravity pulls matter into a ball, although it is not perfectly round, instead being more of an "oblate spheroid" whose spin causes it to be squashed at its poles and swollen at the equator. Roughly 71 percent of Earth's surface is covered by water, most of it in the oceans. About a fifth of Earth's atmosphere is made up of oxygen, produced by plants.

While scientists have been studying our planet for centuries, much has been learned in recent decades by studying pictures of Earth from space . Orbital characteristics Earth spins on an imaginary line called an axis that runs from the North Pole to the South Pole, while also orbiting the sun. It takes Earth 23.439 hours to complete a rotation on its axis, and roughly 365.26 days to complete an orbit around the sun. Earth's axis of rotation is tilted in relation to the ecliptic plane, an imaginary surface through Earth's orbit around the sun. This means the northern and southern hemispheres will sometimes point toward or away from the sun depending on the time of year, varying the amount of light they receive and causing the seasons . Earth's orbit is not a perfect circle, but is rather an oval-shaped ellipse, like that of the orbits of all the other planets. Earth is a bit closer to the sun in early January and farther away in July, although this variation has a much smaller effect than the heating and cooling caused by the tilt of Earth's axis. Earth happens to lie within the so-called "Goldilocks zone" around its star, where temperatures are just right to maintain liquid water on its surface.

Orbit & rotation Some statistics about Earth, according to NASA : Average distance from the sun: 92,956,050 miles (149,598,262 km) Perihelion (closest approach to the sun): 91,402,640 miles (147,098,291 km) Aphelion (farthest distance from the sun): 94,509,460 miles (152,098,233 km) Length of solar day (single rotation on its axis): 23.934 hours Length of year (single revolution around the sun): 365.26 days Equatorial inclination to orbit: 23.4393 degrees Earth's formation and evolution Scientists think Earth was formed at roughly the same time as the sun and other planets some 4.6 billion years ago, when the solar system coalesced from a giant, rotating cloud of gas and dust known as the solar nebula . As the nebula collapsed because of its gravity, it spun faster and flattened into a disk.

Most of the material was pulled toward the center to form the sun. Other particles within the disk collided and stuck together to form ever-larger bodies, including Earth. The solar wind from the sun was so powerful that it swept away most of the lighter elements, such as hydrogen and helium, from the innermost worlds, rendering Earth and its siblings into small, rocky planets. Scientists think Earth started off as a waterless mass of rock . Radioactive materials in the rock and increasing pressure deep within the Earth generated enough heat to melt Earth's interior, causing some chemicals to rise to the surface and form water, while others became the gases of the atmosphere. Recent evidence suggests that Earth's crust and oceans may have formed within about 200 million years after the planet had taken shape. The history of Earth is divided into four eons — starting with the earliest, these are the Hadean, Archean, Proterozoic and Phanerozoic. The first three eons, which together lasted nearly 4 billion years, are together known as the Precambrian . Evidence for life has been found in the Archaean about 3.8 billion years ago, but life did not become abundant until the Phanerozoic.

The Phanerozoic is divided into three eras — starting with the earliest, these are the Paleozoic, Mesozoic, and Cenozoic. The Paleozoic Era saw the development of many kinds of animals and plants in the seas and on land, the Mesozoic Era was the age of dinosaurs, and the Cenozoic Era we are in currently is the age of mammals. Most of the fossils seen in Paleozoic rocks are invertebrate animals lacking backbones, such as corals, mollusks and trilobites. Fish are first found about 450 million years ago, while amphibians appear roughly 380 million years ago. By 300 million years ago, large forests and swamps covered the land, and the earliest fossils of reptiles appear during this period as well. The Mesozoic saw the ascendence of dinosaurs , although mammals also appear in the fossil record about 200 million years ago. During this time, flowering plants became the dominant plant group and continue to be so today. The Cenozoic began about 65 million years ago with the end of the age of dinosaurs, which many scientists think was caused by a cosmic impact.

Mammals survived to become the dominant land animals of today. Composition & structure Atmosphere Earth's atmosphere is roughly 78 percent nitrogen, 21 percent oxygen, with trace amounts of water, argon, carbon dioxide and other gases. Nowhere else in the solar system can one find an atmosphere loaded with free oxygen, which ultimately proved vital to one of the other unique features of Earth — us. Air surrounds Earth and becomes thinner farther from the surface. Roughly 100 miles (160 km) above Earth, the air is so thin that satellites can zip through with little resistance. Still, traces of atmosphere can be found as high as 370 miles (600 km) above the surface. The lowest layer of the atmosphere is known as the troposphere, which is constantly in motion, causing the weather. Sunlight heats the planet's surface, causing warm air to rise. This air ultimately expands and cools as air pressure decreases, and because this cool air is denser than its surroundings, it then sinks, only to get warmed by the Earth once again. Above the troposphere, some 30 miles (48 km) above the Earth's surface, is the stratosphere.

The still air of the stratosphere contains the ozone layer, which was created when ultraviolet light caused trios of oxygen atoms to bind together into ozone molecules. Ozone prevents most of the sun's harmful ultraviolet radiation from reaching Earth's surface. Water vapor, carbon dioxide and other gases in the atmosphere trap heat from the sun, warming Earth. Without this so-called " greenhouse effect ," Earth would probably be too cold for life to exist, although a runaway greenhouse effect led to the hellish conditions now seen on Venus. Earth-orbiting satellites have shown that the upper atmosphere actually expands during the day and contracts at night due to heating and cooling. Magnetic field The northern lights are more formally known as auroras, and are caused by interactions between the solar wind and the Earth's magnetic field. See how the northern lights work in this Space.com infographic . Credit: Karl Tate, SPACE.com Contributor Earth's magnetic field is generated by currents flowing in Earth's outer core. The magnetic poles are always on the move, with the magnetic North Pole recently accelerating its northward motion to 24 miles (40 km) annually, likely exiting North America and reaching Siberia in a few decades.

Earth's magnetic field is changing in other ways, too — globally, the magnetic field has weakened 10 percent since the 19th century , according to NASA. These changes are mild compared to what Earth's magnetic field has done in the past — sometimes the field completely flips, with the north and the south poles swapping places. When charged particles from the sun get trapped in Earth's magnetic field, they smash into air molecules above the magnetic poles, causing them to glow, a phenomenon known as the aurorae , the northern and southern lights. Chemical composition Oxygen is the most abundant element in rocks in Earth's crust, composing roughly 47 percent of the weight of all rock. The second most abundant element is silicon at 27 percent, followed by aluminum at 8 percent, iron at 5 percent, calcium at 4 percent, and sodium , potassium , and magnesium at about 2 percent each. Earth's core consists mostly of iron and nickel and potentially smaller amounts of lighter elements such as sulfur and oxygen. The mantle is made of iron and magnesium-rich silicate rocks.

(The combination of silicon and oxygen is known as silica, and minerals that contain silica are known as silicate minerals.) Internal structure Earth's core is about 4,400 miles (7,100 km) wide, slightly larger than half the Earth's diameter and roughly the size of Mars . The outermost 1,400 miles (2,250 km) of the core are liquid, while the inner core — about four-fifths as big as Earth's moon at some 1,600 miles (2,600 km) in diameter — is solid. Above the core is Earth's mantle, which is about 1,800 miles (2,900 km) thick. The mantle is not completely stiff, but can flow slowly. Earth's crust floats on the mantle much as a wood floats on water, and the slow motion of rock in the mantle shuffles continents around and causes earthquakes, volcanoes, and the formation of mountain ranges. Above the mantle, Earth has two kinds of crust. The dry land of the continents consists mostly of granite and other light silicate minerals, while the ocean floors are made up mostly of a dark, dense volcanic rock called basalt.

Continental crust averages some 25 miles (40 km) thick, although it can be thinner or thicker in some areas. Oceanic crust is usually only about 5 miles (8 km) thick. Water fills in low areas of the basalt crust to form the world's oceans. Earth has more than enough water to completely fill the ocean basins, and the rest of it spreads onto edges of the continents, areas known as the continental shelf. Earth gets warmer toward its core. At the bottom of the continental crust, temperatures reach about 1,800 degrees F (1,000 degrees C), increasing about 3 degrees F per mile (1 degree C per kilometer) below the crust. Geologists think the temperature of Earth's outer core is about 6,700 to 7,800 degrees F (3,700 to 4,300 degrees C), and the inner core may reach 12,600 degrees F (7,000 degrees C), hotter than the surface of the sun. Only the enormous pressures found at the super-hot inner core keep it solid.

Recent exoplanet surveys such as NASA’s Kepler mission suggest that Earth-size planets are common throughout the Milky Way galaxy. Nearly a fourth of sun-like stars observed by Kepler have potentially habitable Earth-size planets . Earth's moon Earth's moon is 2,159 miles (3,474 km) wide, about one-fourth of Earth's diameter. Earth has one moon, while Mercury and Venus have none and all the other planets in our solar system have two or more. The leading explanation for how the moon formed was that a giant impact knocked off the raw ingredients for the moon off the primitive molten Earth and into orbit. Scientists have suggested the impactor was roughly 10 percent the mass of Earth, about the size of Mars. Species overview Earth is the only planet in the universe known to possess life. There are several million known species of life, ranging from the bottom of the deepest ocean to a few miles into the atmosphere, and scientists think far more remain to be discovered. Scientists figure there are between 5 million and 100 million species on Earth , but science has only identified about 2 million of them.

Earth is the only body in the solar system known to host life, although scientists suspect that other candidates — such as Saturn’s moon Titan or Jupiter’s moon Europa — have the potential to house primitive living creatures. Scientists have yet to precisely nail down exactly how complex life rapidly evolved on Earth from more primitive ancestors. One solution suggests that life first evolved on the nearby planet Mars, once a habitable planet, then traveled to Earth on meteorites hurled from the Red Planet. Additional reporting by Nola Taylor Redd, Space.com Contributor Related: Other Planetary Systems l Other Solar Systems HST has observed Beta Pictoris (right) and found the disk to be significantly thinner than previously thought. Estimates based on the Hubble image place the disk's thickness as no more than one billion miles (1600 million kilometers), or about 1/4 previous estimates from ground-based observations. The disk is tilted nearly edge-on to Earth. Because the dust has had enough time to settle into a flat plane, the disk may be older than some previous estimates. A thin disk also increases the probability that comet-sized or larger bodies have formed through accretion in the disk.

Both conditions are believed to be characteristic of a hypothesized circumstellar disk around our own Sun, which was a necessary precursor to the planet-building phase of our Solar Systems, according to current theory. More recent HST observations have shown the disk to be slightly warped as might be expected from the gravitational influence of a planet. This has been confirmed by observations at ESO . Recent observations at radio wavelengths of a gas cloud known as Bok Globule B335 have produced images of material collapsing onto a newly born star (only about 150,000 years old). These observations are helping to understand how stars and planets form. The phenomena observed matches the theory of the formation of the solar system -- that is, a large gas cloud collapsed to form a star with an attendant circumstellar disk in which, over time, planets accreted from the matter in the disk and orbited the Sun . The IRAS satellite found that Vega had too much infrared emission, and that has been attributed to a dust shell (with a mass of maybe Earth's moon). Observations of the very nearby Barnard's Star were once thought to be evidence of gravitational effects of planets but they now seem to have been in error.

The star Gl229 seems to contain a 20 Jupiter mass object orbiting at a distance of 44 AU. An object this large is probably a brown-dwarf rather than an ordinary planet. What may be the first discovery of a planet orbiting a normal, Sun-like star other than our own has been announced by astronomers studying 51 Pegasi, a spectral type G2-3 V main-sequence star 42 light-years from Earth. At a recent conference in Florence, Italy, Michel Mayor and Didier Queloz of Geneva Observatory explained that they observed 51 Pegasi with a high-resolution spectrograph and found that the star's line-of-sight velocity changes by some 70 meters per second every 4.2 days. If this is due to orbital motion, these numbers suggest that a planet lies only 7 million kilometers from 51 Pegasi -- much closer than Mercury is to the Sun -- and that the planet has a mass at least half that of Jupiter . These physical characteristics hinge on the assumption that our line of sight is near the planet's orbital plane. However, other evidence suggests that this is a good bet.

A world merely 7 million km from a star like 51 Pegasi should have a temperature of about 1,000 degrees Celsius, just short of red hot. It was initially thought that it might be a solid body like a very big Mercury but the concensus now seems to be that it is a "hot Jupiter", a gas planet formed much farther from its star that migrated inward. These observations have now been confirmed by several independent observers. And there is some evidence for a second planet much farther out that is not yet confirmed. [ The 5.5-magnitude 51 Pegasi is easily visible in binoculars between Alpha and Beta Pegasi, the western pair of stars in the Great Square of Pegasus. The star's equinox-2000 coordinates are R.A. 22 hours 57 minutes, Dec. +20 degrees 46 minutes. ] On 1/17/96 Geoffrey Marcy andPaul Butler announced the discovery of planets orbiting the stars 70 Virginis and 47 Ursae Majoris.

70 Vir is a G5V (main sequence) star about 78 light-years from Earth; 47 UMa is a G0V star about 44 light-years away. These were discovered using the same doppler shift technique that found the planet orbiting 51 Pegasi. The planet around 70 Vir orbits the star in an eccentric, elongated orbit every 116 days and has a mass about nine times that of Jupiter. Using standard formulas that balance the sunlight absorbed and the heat radiated, Marcy and Butler calculated the temperature of the planet at about 85 degrees Celsius (185 degrees Fahrenheit), cool enough to permit water and complex organic molecules to exist. The star 70 Vir is nearly identical to the Sun, though several hundred degrees cooler and perhaps three billion years older. The planet around 47 UMa was discovered after analysis of eight years of observations at Lick Observatory . Its period is a little over three years (1100 days), its mass about three times that of Jupiter, and its orbital radius about twice the Earth's distance from the Sun. This planet too probably has a region in its atmosphere where the temperature would allow liquid water.

As of April 1996, Drs. Marcy and Butler have discovered yet another planet this time around the star HR3522 (aka Rho 1 Cancri, 55 Cancri) about 45 light years from the Earth. The planet is estimated to be about 0.8 Jupiter masses. It is likely that several more planets will show up in the initial set of 120 stars that they have monitored. Several more extra-solar planets have now been discovered by the Butler/Marcy method. It seems likely that there are a very large number of such planets out there. Another extra-solar planet has been discovered orbiting 16 Cygni B. But unlike all other previously known planets this one has a very large orbital eccentricity (0.6); its orbit carries it from a closest distance of 0.6 AU from its star to 2.7 AU. This calls into question many theories of planetary formation. Detecting extra-solar planets directly is very difficult. Even the Hubble Space Telescope wouldn't be able to image planets at the expected sizes and distances from their suns.

What HST did find were disks of matter around stars seen in silhouette against the Orion Nebula (called 'proplyds', for 'proto-planetary disks' (right). This is great evidence for how common these objects are, but the scale is way too small to say anything directly about planets there. More detailed HST images are now available, too. Nevertheless, it might be possible to detect the infra-red radiation of very large planets (Jupiter-sized or more) in some circumstances. By a stroke of good luck, HST has taken an image of what appears to be a planet escaping from a double star system. See the 1998 May 28 announcement . If this is confirmed, the existence of extrasolar planets will be undeniable.

More about other planetary systems The Search for the Extrasolar Planets : A Brief History of the Search, the Findings and the Future Implications (very comprehensive!) NASA's Exploration of Neighboring Planetary Systems (ExNPS) Program home page the Darwin Project , an ESA proposal for space-based infrared interferometry Kepler , a low cost mission to detect Earth-like planets Open Issues It is likely that more planetary systems will be discovered using the methods that found 51 Pegasi, 70 Virginis and 47 Ursae Majoris. Exciting times are ahead! The 51 Pegasi planetary system is quite different from our solar system. But the 70 Virginis and 47 Ursae Majoris systems appear to be more "normal". With several known planetary systems it is now possible to make more general statements and to better test theories of planetary formation. None of the extra-solar planets discovered so far are at all similar to the Earth, as expected given our current methods. How can we get more evidence about extra-solar Earth-like planets? Will the ExNPS program get funded? How do you get a planet with a large eccentricity? [abstract of the above paper by Dr.

Fraknoi]: Interest among astronomers in the detection of extra-solar planets is accelerating with the growing realization that it may soon be technically feasible. The ongoing renaissance in telescope construction and the anticipated launches of new space platforms are encouraging many scientists to review and improve the means by which planets can be discovered. The direct detection of the light from a distant planet would be the most compelling means of discovery and to gauge the feasibility of various search strategies, astronomers have traditionally used the current Jupiter as a benchmark planet. However, in principle, extra-solar giant planets (EGPs) can have a wide range of masses and, hence, can be significantly brighter than Jupiter. Furthermore, the maximum mass a planet can have is not known a priori, and observations will be needed to determine it. We predict the optical and infrared fluxes of EGPs that searches in the next few years may reveal.

Question: Which planet has a mass which is over three hundred times that of the Earth?
Answer: JUPITER